In recent years, projection display apparatuses (projectors) using various kinds of light modulation elements have received attention as projection imaging devices capable of providing large size display. These projection display apparatuses illuminate a light modulation element capable of being optically modulated by a transmission or reflection liquid crystal, a DMD (digital micro-mirror device) capable of changing a reflection direction by very small mirrors arranged in the form of an array, or the like with light emitted from a light source as a light generating instrument, form an optical image corresponding to an image signal from the outside on the light modulation element, and project, at an enlarged scale, an optical image being illuminating light modulated by the light modulation element onto a screen by a projection lens.
As important optical characteristics of the projected large size image, there are a brightness of light emitted from the projection lens, a uniformity of brightness, a color reproducibility, i.e. a capability of more faithfully reproducing single colors such as red, green and blue, and colors such as white obtained by chromatic synthesis of the three colors, and the like.
In addition, recently, as a projection display apparatus, comprehensive capabilities required as a general image display apparatus, such as an instantaneous lighting capability of reducing time taken until the brightness of an image displayed on a screen reaches a maximum brightness after the power is tuned on, an easiness of installation, and a portability for conveyance or the like, have received attention as important items.
A conventional projection display apparatus using a light source unit 403 using a white lamp 401 such as an ultra-high pressure mercury lamp, an illumination unit 35 formed using an optical instrument allowing uniform illumination, a reflection display element 41 as a light modulation element and a projection lens 51 is shown in FIG. 8.
As an optical instrument allowing uniform illumination, a hallow cylindrical rod integrator 32 formed from a glass column or laminated mirrors is used. In this rod integrator 32, light incident from an opening on the incidence side is totally reflected and reflected at the mirror surface repeatedly to propagate through the rod, and a uniform light flux is emitted from an opening on the exit side. Furthermore, by using an illumination unit 35 using an optical instrument such as a lens 33, a mirror and a prism 36 in combination, a highly uniform light flux can be illuminated onto the reflection display element 41.
It is known that uniform illumination onto the display element can also be performed by using a lens array having a plurality of lenses arranged two-dimensionally as an optical instrument allowing uniform illumination.
Here, an optical system using the illumination unit 35 by the rod integrator 32 is shown in the figure, and the entire optical system of the projection display apparatus is described.
Light emitted from the lamp 401 as an optical instrument is collected at a reflector 402 which is light collecting instrument. A light flux emitted from an opening of the reflector 402 at this time is a light flux having a large difference in luminance between an area near the center of the light flux and a peripheral area. Then, a uniform flux is emitted from an opening on the exit side due to the rod integrator 32 described above. The light flux emitted from the rod integrator 32 propagates light to a position at which the reflection display element 41 capable of forming an image by light modulation, by the illumination unit 35 such as the lens 33, the mirror and the prism 36, such that the light becomes a light flux having a size suitable for an effective region of the reflection display element 41.
Traditionally, the white lamp 401 used as a general light source emits white light, but if white light illuminates the reflection display element 41 and a light flux modulated by the reflection display element 41 is projected onto a screen via the projection lens 51, only images of white and black, i.e. gray scales are output. Thus, in the case that color images are to be displayed, it is necessary to separate white light into three primary colors of red, green and blue and chromatically synthesize light fluxes of three colors again.
Thus, white light emitted from the white lamp 401 is separated into three primary colors of light by illuminating the display element with colors of red, green and blue in a time sequence by rotating a color separation filter called a color wheel 411 in a predetermined cycle within a period for display of one image, and images of respective colors formed by one reflection display element 41 are projected onto a screen during a period for illumination with light of respective colors to realize a color image. In FIG. 8, the color wheel 411 is inserted between the lens 31 and the rod integrator 32 as a color separation filter 21.
In this projection display apparatus, an image displayed within a period for formation of one screen (about 17 milliseconds for image display of NTSC and the like) produces an illusion as if images of different colors glittered at the same time because light caught by eyes is recognized or a certain time even if the image is an image displayed with different colors, and thereby a color image can be displayed.
In this way, a color image formed by the reflection display element 41 is displayed on a screen in a large size, brightly and highly uniformly.
In recent years, in the above-mentioned conventional optical system, instead of the white lamp 401 using mainly an ultra-high pressure mercury lamp, a projection display apparatus formed using a light source, called a solid light source such as a light emitting diode 1, emitting single-color light as shown in FIG. 10, or the like, is known (e.g. see “Performance of High Power LED Illuminators in Color Sequential Projection Displays”; Gerard Harbers, et at al. IDW'03 pp 1585-1588). The projection display apparatus shown in FIG. 10 is comprised of a light source unit 4 comprising a red light emitting diode 1(a) and a lens 2(a) of collecting light fluxes emitted from the light source, a green light emitting diode 1(b) and a lens 2(b) of collecting light fluxes emitted from the light source, a blue light emitting diode 1(c) and a lens 2(c) of collecting light fluxes emitted from the light source, a cross prism 3 of synthesizing light fluxes of the light sources, an illumination unit 35 using lenses 31, 33, 34 allowing a light flux to be shaped and uniformed according to an illumination region, a rod integrator 32 allowing highly uniform illumination, and a prism 36 guiding light transmitted through the lens 34 to a reflection display element 41, the reflection display element 41 as a light modulation element modulating illuminating light, and a projection lens 51.
For solid light sources such as light emitting diodes 1(a) to 1(c) emitting single-color light, it is known that startup time taken until almost all light outputs corresponding to a power are emitted after the power is supplied, or startup time taken until almost all light outputs no longer exist after the supply of power is stopped is 1 microsecond or less, which is very short compared to the conventional white lamp 401. Namely, the light emitting diode has an advantage that the switching between light-up and light-out can be done instantaneously.
In addition, the light emitting diode can emit single-color light, and therefore it is unnecessary to take the trouble to chromatically separate emitted light. Thus, as shown in the light emitting diodes 1(a) to 1(c) shown in FIG. 10, light emitting diodes emitting red light (having a wavelength of about 600 to 700 nm), green light (having a wavelength of about 500 to 570 nm) and blue light (having a wavelength of about 430 to 490 nm), respectively, are used as light sources, and each diode is lighted up and lighted out repeatedly in a predetermined cycle under control from a control instrument (not shown), whereby a color image can be displayed as in the projection display apparatus of FIG. 8. It is known that this projection display apparatus does not require the color separation filter 21 such as the color wheel 411 for color separation used in the optical system having the conventional white lamp 401 as a light source, thus making it possible to form a projection display apparatus having a further optical system.
The above described projection display apparatus having, as a light source, solid light sources such as light emitting diodes 1(a) to 1(c) has the problems described below.
That is, in the projection display apparatus shown in FIG. 10, it is desired that a white color made by synthesizing three colors of red, green and blue should be adjusted so as to obtain light having a white color on a trail of black body radiation at a color temperature of 5000 to 10000 K, or very near the trail, and a white color significantly deviated from this range degrades the quality of a projected image. In this way, in light having a white color on a trail of black body radiation at a color temperature of 5000 to 10000 K, or very near the trail, the ratio of the radiant quantities of red, green and blue is often approximately 1:1:1 although it more or less varies depending on the main wavelength and the spectral bandwidth of a light source used. However, red light, green light and blue light are mutually different in brightness sensed by naked eyes. Generally, if the ratio of red light, green light and blue light having the same radiant intensity is represented by a ratio of brightness sensed by humans (hereinafter referred to as light amount), it is often red: green: blue=about 3:7:1, for example. Thus, when the white color is balanced, it is preferable that the ratio of the light amounts is, for example, red: green: blue=about 3:7:1.
On the other hand, there is a problem as described below.
The light amount of a light emitting diode emitting light from light emitting portions of almost same size, which is commercially available from Lumileds Co., Ltd. (U.S.), which is one of manufactures of light emitting diodes that can currently emit maximum outputs, is about 44 lumens for red, about 80 lumens for green and about 18 lumens for blue, and the ratio of the light amounts is red: green: blue=about 2:4:1 in which the light amounts of red and green light are small, and thus it does not coincide with the allocation ratio described above.
Thus, for light emission by such a light emitting diode, almost unique adjustment of the light amount is required in color synthesis, and an appropriate white color is obtained by adjusting the light amount in the following way.
A first control method adjusts the light intensities (referring to the momentary light amount as in the description below) of light emitting diodes of respective colors as shown in FIG. 11. Specifically, control is performed so that the light emitting diode of green is made to emit light at a maximum intensity, while the light intensities of the light emitting diode of red and the light emitting diode of blue are each lower than the maximum light intensity. The periods of light emission for red, green and blue light emitting diodes in FIG. 11 are the same with the period T for display of one image (about 17 milliseconds for image display of NTSC) divided into three equal periods. Under this condition, the light amounts of respective light are represented by the areas (products of light intensities and light emission periods) of a region 501 of the red light emitting diode 1(a), a region 502 of the green light emitting diode 1(b) and a region 503 of the blue light emitting diode 1(c), and the ratio thereof gives an allocation ratio allowing for a specific sensitivity of naked eyes.
However, in the adjustment shown in FIG. 11 in which the light emission period is fixed and the light intensity is made variable, the light intensity of the green light emitting diode 1(b) is determined to be a maximum light intensity and on the basis thereof, the light intensities of other light emitting diodes are determined. Thus, the maximum light intensity of the green light emitting diode 1(b) restricts the light intensities of all the light emitting diodes, and it is difficult to further increase the light amount in a state in which a high color reproducibility of white light is attained.
The value of the maximum light intensity of each color is a maximum light emission intensity obtained under conditions such as the amount of current within the range not destroying the light emitting portion of the light emitting element, product specifications, and the temperature requirement and the amount of current to be met for prolonging the lifetime.
Thus, a second control method described below is carried out. Control is performed so that all the light emitting diodes of red, green and blue are made to emit light at a maximum light intensity, while each light emitting diode is made to have a different light emission period and the green light emitting diode with a smaller light amount is made to have a longer light emission period, as shown in FIG. 12. Specifically, control is performed so that in a period T for display of one image, a light emission period Gt for the green light emitting diode is longer than one third of the period T for display of one image, light emission periods Rt and Bt for other light emitting diodes are shorter than the light emission period Gt (the light emission period for the blue light emitting diode is shorter than the light emission period for the red light emitting diode). As in FIG. 11, the light amounts of respective light sensed by naked eyes are represented by the areas of a region 511 of the red light emitting diode, a region 512 of the green light emitting diode and a region 513 of the blue light emitting diode, and the ratio thereof gives an allocation ratio (e.g. 3:7:1) allowing for a specific sensitivity of naked eyes.
For the example shown in FIG. 11 and the example shown in FIG. 12, the ratio of the areas (light amounts) for red, green and blue is the same, but the absolute value, i.e. the area of the regions (light amount) is greater in FIG. 12. Thus, a larger light amount can be obtained while the allocation ratio of respective colors is maintained.
However, in the adjustment shown in FIG. 12 in which the light emission period is made variable and the light intensity is fixed, it is the green light that is the greatest in light amount among red, green and blue colors as described above, and therefore if the lighting period for the green light emitting diode is prolonged for increasing the light amount of green in order to increase the brightness of emitted light as the projection display apparatus, the white color becomes a greenish white color. That is, lighting over a period longer than a predetermined lighting period has a problem of degradation in color reproducibility for the white color.
As described above, in a light source using a solid light source, such as light emitting diodes, capable of emitting single-color light, it is difficult to increase the light amount and also maintain a color reproducibility.
The present invention has been made in view of the above problems, and its object is to obtain a light emission method of a light source and a light emitting apparatus capable of increasing the light amount while maintaining a color reproducibility, a projection display apparatus using the same, and the like.